Traditional IQ scores cannot provide a meaningful assessment of how intelligent a person is, it is claimed.

To investigate the value of IQ, scientists conducted an online study which attracted more than 100,000 participants from around the world.

Volunteers were asked to complete 12 tests of reasoning, planning, memory, and attention, and to provide personal details about their background and lifestyle.

The results, published in the journal Neuron, suggest that intelligence is too complex to be represented by a single number.

When a wide range of cognitive abilities were probed, variations in performance could only be explained by examining at least three separate factors. These were short-term memory, reasoning, and a verbal component.

Scanning studies showed that each of these three elements activated a different brain circuit.

Study leader Dr Adrian Owen, a British neuroscientists based at Western University in Canada, said an "astonishing" number of people contributed to the research.

"We expected a few hundred responses, but thousands and thousands of people took part, including people of all ages, cultures and creeds and from every corner of the world," he said.

The test went viral after being launched online in the magazines Discovery and New Scientist and The Daily Telegraph newspaper.

The results also provided a wealth of new information about how facts such as age, gender and playing computer games influence brain function.

"Regular brain training didn't help people's cognitive performance at all, yet ageing had a profound negative effect on both memory and reasoning abilities," said Dr Owen.

People who regularly played computer games performed significantly better in both reasoning and short-term memory tests, the study found.

Smokers did badly in tests of short-term memory and verbal ability while people prone to anxiety had particularly poor short-term memories.